Scientists discover the missing link that brings mercury into the food chain

A team of US scientists has revealed that there is a link between the quantity of algae in water and the amount of mercury moving into the food chain, suggesting that algae – in combination with pollution and other sources of the metal - is responsible for the levels of mercury in fish.


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Scientists from Dartmouth College in New Hampshire, studied the movement of mercury, in the form of methylmercury, from the water into algae, and then as it was taken up by Daphnia – a food source for many species of fish. Neither fish nor Daphnia are affected by methylmercury. However, they biomagnify the pollutant, causing problems for any people who eat the fish.

Last year it was revealed that one in ten women in the US are at risk of having children with neurological problems due to mercury intake (see related story).

The study found that where there is a large amount of algae present in a water body the methylmercury becomes widely dispersed throughout it. However, where only a small amount of algae is present the methylmercury becomes much more highly concentrated, causing the daphnia that feeds on it to take up much more of the pollutant. This might also explain why marine systems – in which algae is much more scarce – are so susceptible to methylmercury uptake by fish such as yellowfin tuna and sharks.

“We suspected there was an algal link, but few laboratories had the technology to make such precise measurements before,” said Paul Pickhardt, senior author of the research paper, and a graduate student at Dartmouth College. “With our trace-metal techniques, we’ve achieved mercury detection levels that are 50 times more sensitive than any other method.”

On the other hand, the researchers found that ordinary inorganic mercury does not biomagnify, which, the researchers claim, would explain why methylmercury – the less common form of mercury – is the form in which mercury is most usually found in fish, even if it is barely detectable in the water.

“These results tell us that over the season in a lake, changes that cause the algae to increase or decrease, can also quickly produce changes in the amount of mercury that moves through the ecosystem,” said Carol Folt, co-author of the research, and Director of Dartmouth College’s Center for Environmental Health Sciences. “This is important because right now. Scientists and government officials are trying to figure out how and when to measure mercury in order to issue more precise advisories about human consumption of fish.”

However, more research needs to be done, said Celia Chen, the third co-author of the research, and Research Assistant Professor of Biological Sciences at Dartmouth. Scientists need to better understand the intricacies of how metals move through the food chain. The team will now be completing an intensive study of four lakes with high and low algal systems.

The research will be published in the 2nd April issue of the Proceedings of the National Academy of Sciences.

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